Not Applicable
Not Applicable
The present invention relates to optical telecommunications channel routers, and in particular to a narrowband optical channel router using a waveguide-coupled microcavity optical resonator.
There is a growing demand for increasing data transmission capacity in optical communications networks, i.e. for greater bandwidth. WDM (wavelength-division-multiplexing) systems provide a method for increasing the capacity of existing fiber optic links without physically modifying the optical fiber, by allowing multiple wavelengths to be transmitted and received over a single optical fiber. Dense WDM (DWDM) systems can be utilized to further increase information transmission capacity. DWDM systems increase the capacity of an embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated wavelength band, and then multiplexing the resulting signals onto one fiber. DWDM systems combine multiple optical signals so that they can be amplified as a group, and transported over a single fiber to increase capacity. Each signal can be carried at a different rate, and in a different format. DWDM systems can multiplex and demultiplex large numbers of discrete communication channels onto a single optical fiber, and transmit these channels over long distances.
Channel routing and switching is an important function performed by DWDM network components, and allows service providers to have optical access to data at desired nodes on the network. For example, selected channels may be extracted or xe2x80x9cdroppedxe2x80x9d from a multiplexed signal, and routed to desired nodes. Alternatively, extracted signals, or newly generated signals, may be inserted or xe2x80x9caddedxe2x80x9d into the multiplexed signal. This extracting and inserting of optical signals is generally referred to as add/drop multiplexing, and is carried out with channel routers, typically optical wavelength add/drop (OWAD) filters.
Resonant filters are attractive candidates for performing these channel routing functions in DWDM networks, because resonators can potentially realize the narrowest linewidth for a given device size. For optimal performance, it is desirable that resonators be characterized by low loss, large free-spectral range, and processing control over spectral characteristics such as filter linewidth and resonant frequency.
During the past few years, a substantial amount of research has been performed in the field of optical microcavity physics, in order to develop high cavity-Q optical microcavity resonators. Optical microcavity resonators have quality factors that are several orders of magnitude better than typical surface etched resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. The result is a clean, smooth silica surface with low optical loss and negligible scattering. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
For these optical microcavity resonators, measured Qs as large at 1010 have been reported, whereas commercially available devices typically have Qs ranging from about 105 to about 107. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities. Because the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. The use of SPARROW waveguides for efficiently coupling light into the microcavity resonant modes is described in detail in U.S. patent application Ser. No. 09/893,954 (hereinafter the xe2x80x9c""954 applicationxe2x80x9d), entitled xe2x80x9cOptical Microcavity Resonator System,xe2x80x9d which is commonly owned by the present assignee and which is incorporated herein by reference. As a result of their small size and high cavity Q, microcavity resonators have the potential to provide superior performance in numerous applications, including for example applications such as optical DWDM communications systems that call for ultra-narrow linewidths.
Techniques used in the prior art for channel routing in DWDM systems include optical grating techniques and fused coupler techniques. These techniques require, at a minimum, channel spacings of about 25 GHz. The number of communication channels that can be packed within the same transmission wavelength range would be increased substantially, by reducing the channel spacing. The narrow bandwidth of the resonance modes coupled into microcavity resonators provides the potential for decreasing the channel spacings in WDM and DWDM networks by several orders of magnitude.
It is therefore desirable to effectively implement optical microcavity resonators in channel routing applications in optical telecommunications.
The present invention provides an optical channel router, based on microcavity resonator technology. In particular, the present invention features SPARROW (Stripline-Pedestal Anti-Resonant Reflecting Optical Waveguide) coupler structures for coupling optical radiation from a waveguide channel into and out of high-Q optical microcavity resonators. The narrow bandwidth of microcavity WGM (whispering gallery mode) resonances (xcx9c1 MHz) allows for in-line filtering and routing of narrowband optical communications channels. As a result of the high cavity Qs of the WGMs propagating within the microcavities, an optical channel add/drop filter as described in the present disclosure can be used to perform ultra-high resolution separation of closely spaced, narrowband optical communications channels in a ultra-dense wavelength division multiplexing network (UDWDM).
In one embodiment, the present invention features an optical channel add/drop router that includes a substrate, a first and a second optical waveguide disposed on the substrate, and at least one optical microcavity resonator. Each optical waveguide has a SPARROW structure, composed of a multi-layer dielectric stack including alternating high and low refractive index dielectric layers, and a waveguide core disposed on the dielectric stack. The first optical waveguide defines a throughput channel between a first I/O port and a second I/O port. The second optical waveguide defines an add/drop channel between an originating end of the waveguide and an add/drop port.
The microcavity resonator is disposed on the substrate and at a distance from the optical waveguide that is sufficiently small so as to allow evanescent coupling between the microcavity and the light propagating along the waveguide. When the frequency of the light propagating along one of the waveguides is in resonance with a whispering gallery mode of the microcavity, light is coupled into the microcavity and then out of the microcavity onto the remaining one of the optical waveguides.
The channel add/drop router device can function both as a channel add filter, or a channel drop filter. When the channel router operates as a channel add filter, the add/drop port of the second waveguide functions as an add port, into which a beam of light, containing at least one frequency component that matches a resonant mode of the microcavity, is inputted. The matching frequency components are coupled into the microcavity, then out of the microcavity to be added onto another beam of light propagating through the throughput channel in the first waveguide.
When the channel router operates as a channel drop filter, one of the I/O ports of the first waveguide functions as an input port, and receives a beam of light that includes a plurality of frequency components, and that propagates along the throughput channel. Frequency components within the beam that do not match any resonant mode of the optical microcavity propagate through the throughput channel without coupling into the microcavity. Frequency components within the beam that substantially match one or more resonant modes of the optical microcavity are coupled into the microcavity, and are subsequently coupled out of the microcavity onto the drop channel, to be output from the drop port. In this way, the matching frequency components are xe2x80x9cdroppedxe2x80x9d from the input beam of light.
Optical transfer efficiencies of more than 95% has been attained at resonant linewidths of about 1 MHz-100 MHz, using the channel add/drop router of the present invention. A multiple channel add/drop router device can be constructed, by arranging multiple microspheres in series on a single waveguide chip.